A bolting structure of a sub-frame includes left and right reinforcing members 20 and 30 each formed at one side and the other side of a side member 10 in a transversal direction, and inner and outer pipes 40 and 50 each formed at an inner side and an outer side of the left reinforcing member 20. The bolting structure further includes a support bracket 70 formed at the inner side of the left reinforcing member 20 and supporting the inner pipe 40, and a bolt 60 bolted to the inner pipe 40 through the outer pipe 50 in a state in which a sub-frame (not shown) is inserted into the outer pipe 50, thereby bolting the sub-frame, as shown in FIGS. 1 and 2.
Here, the inner pipe 40 is supported by the support bracket 70 to thereby increase rigidity of the inner pipe 40. That is, the bolting structure of a sub-frame according fixes the sub-frame through the left reinforcing member 20 of which one end is supported by the side member 10.
However, in the bolting structure of a sub-frame, the left reinforcing member 20, to which the sub-frame is fixed, has weak rigidities in a front and rear direction and in a left and right direction, and is deformable at the time of having a transversal load applied thereto.
Furthermore, a vibration load transferred through the sub-frame is transferred to a vehicle body through the reinforcing members, and the vibration load transferred to the vehicle body generates noise such as road noise, booming noise, and acceleration penetration sound in a vehicle.